T1 is a standard for digital transmission in North America and is used primarily to provide telephone services across remote distances. The T1 digital transmission link has a capacity of 1.544 Mbps (1.544 mega bits per second). In most T1 telecommunications systems, 24 voice channels (conversations) are sampled at an 8 kHz rate, with 8 bits per sample (each 8 bit conversation sample is termed a "DS0"). The twenty-four 8 bit samples are time division multiplexed (TDM) together to create a frame (a "DS1"). To identify individual time slots within a TDM frame, a framing bit having an identifiable data sequence patterned thereon is appended to each frame. The frames are commonly strung together in various formats to create superframes. The Superframe (SF) and Extended Superframe (ESF) formats are two well known examples of widely used framing formats.
Existing telecommunications systems are mostly comprised of copper wire in the subscriber loop. Today, however, many passive distribution media are becoming more viable as an alternative to copper wire. For example, maturing fiber optic technology is making it possible to cost effectively deploy optical fiber in the subscriber loop. One such system employing optical fiber is disclosed in U.S. Pat. No. 4,977,593 to Ballance, issued Dec. 11, 1990, and assigned to British Telecommunications, which is incorporated herein by reference. Optical fiber as a transmission medium provides numerous advantages. Optical fiber provides a noise-free signal transmission environment, is resistant to electromagnetic interference, generates no electromagnetic interference which could initiate crosstalk, and supports very high transmission rates. In addition, optical fiber offers additional potential for future upgrades such as supplying broadband services which are not transmittable over copper wire.
However in systems employing passive distribution networks, the signals are reframed using a modified framing format. In so doing, each DS0 is no longer associated with its DS1 framing bit. A different mechanism for conveying multiframe and alignment information is therefore required.